1. Field of the Invention
The present invention relates to industrial fluids. More particularly, the present invention relates to improved hydraulic fluids that exhibit oxidative stability, ready biodegradability, low volatility, and a high viscosity index.
2. Description of Related Art
In recent years, there has been a strong trend in the U.S. and Europe to develop a readily biodegradable, low volatility, and high viscosity index industrial fluid. This desire for an environmentally friendly natural ester fluid is driven by a variety of factors, including a belief in the green movement that natural ester fluids are renewable resources that have less impact on the carbon cycle balance and a belief that the biodegradability of these fluids makes disposal costs less of an issue
Additionally, there has been a drive in the metalworking industry to dramatically lower the threshold limit value (TLV) for mineral oil mists. Although there is currently no substantive evidence that oil mist exposure has any effects on machinists' long term respiratory health, the American Conference of Governmental Industrial Hygienists (ACGIH) has proposed a TLV of 0.2 mg/m3 which is a 25-fold reduction from the previous TLV of 5 mg/m3 (see J. A. Bukowski, Applied Occupational and Environmental Hygiene, 18:828-837 2003)). With such increased pressure to remove oil from the workplace, there is a corresponding increase in interest in finding alternative basestocks.
A number of articles have been published showing it to be feasible to use canola and rapeseed oils as industrial fluids. Owing to their low oxidative stability, however, a large amount of antioxidant is required to protect these vegetable oils, which precludes their widespread use in industry. In particular, polyunsaturate levels above 2-3% result in polymerization cross-linking, as well as oxidative and biological degradation during product use. Additionally, these glycerides are hydrolytically unstable for most applications where lubricant life is expected to be weeks, months, or years. Typically, unless severe steps are taken to control microbial growth, the monounsaturates (e.g., oleates) are biodegraded far too rapidly for use in emulsion applications.
Attempts have been made to reduce the amount of polyunsaturated fatty material, and thereby the instability, by increasing the oleic acid content, either by hydrogenation followed by fractionation or by genetically engineering the crops. For example, in the production of high erucic (oleic) acid rapeseed (HEAR) oil, the percentage of two and three double bond fatty acids (i.e., linoleic and linolenic) is reduced to very low levels. As a result, HEAR oil possesses a high oxidative stability, which produces smaller deposits upon heating. Unfortunately, the extra processing that is necessary to improve the performance can more than double the cost of these products.
U.S. Pat. No. 6,531,429 discloses compositions comprising thiophosphoric acid esters and dithiophosphoric acid esters or phosphoric acid thioesters and oil additives from the group of the polyol partial esters, amines and epoxides, and also to the use of those lubricant compositions in improving the performance properties of lubricants, such as greases, metal-working fluids, gear fluids or hydraulic fluids. Thiophosphoric acid esters and dithiophosphoric acid esters or phosphoric acid thioesters are present in the compositions preferably in a concentration of less than 400 ppm.
U.S. Pat. No. 6,583,302 discloses the modification of triglyceride oils having unsaturated fatty acid substituents to convert sites of unsaturation to C2 to C10 diesters. The resulting derivatives are said to be characterized by thermal and oxidative stability, have low temperature performance properties, are environmentally-friendly, and have utility as hydraulic fluids, lubricants, metal working fluids and other industrial fluids. The triglyceride oils are most easily prepared via epoxidized vegetable oils which are converted to the diesters in either a one- or two-step reaction.
Flider, F. J., INFORM 6(9):1031-1035 (September, 1995) reported that although there is no one universal vegetable oil that can be used in all lubricant applications, both HEAR oil and canola oil economically and efficaciously meet the requirements of a broad cross-section of the lubricants industry. The author predicted that through continuing advances in traditional plant breeding and genetic engineering, an even wider range of rapeseed oils with functionalities and performance characteristics that meet the burgeoning demands of the lubricant industry would be developed.
Wu, X. et al., JAOCS 77(5):561-563 (May, 2000) described the application of epoxidized rapeseed oil as a biodegradable lubricant. They found that epoxidation treatment had no adverse effect on the biodegradability of the base stock and that the epoxidized oil had superior oxidative stability compared to rapeseed oil based on the results of both oven tests and rotary oxygen bomb tests and better friction-reducing and extreme pressure abilities according to tribological investigations. Moreover, the oxidative stability could be dramatically promoted by the addition of a package of antioxidants. Formation of a tribopolymerization film was proposed as an explanation of the tribological performance of epoxidized rapeseed oil.
Adhvaryu, A. et al., Industrial Crops and Products 15:247-254 (2002) demonstrated the improved performance of epoxidized soybean oil (ESBO) over soybean oil (SBO) and genetically modified high oleic soybean oil (HOSBO) in certain high temperature lubricant applications. They validated the thermal and deposit forming tendencies of these oils using micro-oxidation and differential scanning calorimetry in conjunction with identification of oxidized products by infrared spectroscopy and also discussed the function of phenolic antioxidants in these oils. Boundary lubrication properties under high load and low speed were determined and the variations explained based on the structural differences of these vegetable oils.
The disclosures of the foregoing are incorporated herein by reference in their entirety.